1. Field of the Disclosure
The present disclosure relates to a mobile device used in a keyless entry system in which wireless communication is performed with a vehicle-mounted device, and more particularly to a mobile device that has a function by which communication is automatically performed with a vehicle-mounted device to start an engine in the vehicle.
2. Description of the Related Art
Recently, so-called keyless entry systems are being used for automobiles and other moving vehicles. With a keyless entry system, when the user operates the switch unit of a mobile device, a communication signal is transmitted from the mobile device to a vehicle-side device and the doors are locked or unlocked without the user having to insert a key into a keyhole in a door. More recently, so-called passive keyless entry systems are being used. With a passive-keyless entry system, when, for example, the user touches the vehicle with a hand in a state in which the user is carrying a mobile device, the doors are automatically locked or unlocked without the user having to operate a switch of the mobile device.
Conventionally, to start an engine, a comparison is made of physical engagement of a key inserted into a keyhole. To improve convenience, however, recently so-called immobilizer systems are being used, which allow an engine to be started through an electronic comparison instead of a physical comparison.
Japanese Unexamined Patent Application Publication No. 2002-322841 discloses a mobile device adaptable to a keyless entry system that has an immobilizer system, as described above, that is used to start an engine. FIG. 7 illustrates the structure of the mobile device (electronic key) 912 described in Japanese Unexamined Patent Application Publication No. 2002-322841.
The electronic key 912 includes an ID code control unit 929 and a transponder 922. The ID code control unit 929 includes a reception circuit 920, a microcomputer 921, a transmission circuit 923, a coil antenna 924, an antenna 925, and a battery 926. The reception circuit 920 and transmission circuit 923 are connected to the microcomputer 921. The coil antenna 924 is connected to the reception circuit 920. The antenna 925 is connected to the transmission circuit 923. The reception circuit 920 receives a request signal from a transmission circuit in a vehicle-side device through the coil antenna 924. The transmission circuit 923 transmits an answer signal to a reception circuit in the vehicle-side device through the antenna 925.
The transponder 922 includes a coil antenna 935, a power circuit 936, and a transponder control circuit 937, in which a non-volatile memory 937a is included. The power circuit 936 is connected to the transponder control circuit 937. The coil antenna 935 is connected to the power circuit 936. The power circuit 936 receives an electromagnetic field from a vehicle-side immobilizer through the coil antenna 935 and produces electric power from the received electromagnetic field. The power circuit 936 then supplies the produced electric power to the transponder control circuit 937. The transponder control circuit 937 uses the electric power supplied from the power circuit 936 to perform frequency modulation (FM) on a transponder signal including a transponder code so that the transponder signal is converted to a transponder radio wave. The transponder control circuit 937 then transmits the transponder radio wave to the vehicle-side device through the coil antenna 935.
The electronic key 912 structured as described above can perform transmission and reception to and from the vehicle-side device through the coil antenna 924 and antenna 925, which are disposed in the electronic key 912. The electronic key 912 can also perform transmission and reception to and from the vehicle-side immobilizer through the coil antenna 935.
However, the electronic key 912 described in Japanese Unexamined Patent Application Publication No. 2002-322841 has the problem described below.
When a mobile device that is adaptable to an immobilizer system and is similar to the electronic key 912 is formed on a circuit board, a mobile device (electronic key) 812 having a structure as illustrated in FIG. 8 is possible. The mobile device 812 includes an ID code control unit 829, a transponder 822, and an antenna 825. With the mobile device 812, the antenna 825, which is used to transmit an answer signal to a vehicle-side device, is preferable a loop antenna because it can be efficiently formed on the circuit board.
The ID code control unit 829 is formed by mounting a microcomputer 821, a transmission circuit 823, a battery 826, other circuits (not illustrated), and the like on a circuit board 809. The transponder 822 is formed by mounting a coil antenna 835, a transponder control circuit 837, other circuits (not illustrated), and the like on the circuit board 809. The microcomputer 821 is shared by the ID code control unit 829 and transponder 822.
The antenna 825, which is connected to the transmission circuit 823, is formed by a conductive wire 825a such as a copper foil pattern, the conductive wire 825a being formed on the circuit board 809. The opening of the antenna 825 needs to be widened as much as possible to obtain a necessary antenna gain. Therefore, the antenna 825 is preferably formed along the outer shape of the circuit board 809.
The coil antenna 835 for use by the transponder 822 has a substantially rectangular shape. Since the coil antenna 835 performs transmission and reception to and from a vehicle-side immobilizer, the distance between the immobilizer and one shorter edge of the rectangular shape needs to be shortened as much as possible. Therefore, the coil antenna 835 is preferably disposed near the edge, of the circuit board 809, that faces the vehicle. If a metal is present near the coil antenna 835 for use by the transponder 822, the Q value of the coil antenna 835 may be lowered and its communication performance may thereby be lowered. If the coil antenna 835 is disposed near the antenna 825, mutual coupling occurs between them, making the flow-in of a disturbing signal likely to occur. In view of this, the coil antenna 835 needs to be disposed away from the antenna 825, which is formed by the conductive wire 825a such as a copper foil pattern, so a certain distance is reserved between the coil antenna 835 and the antenna 825.
As a result, one shorter edge of the rectangular shape of the coil antenna 835 is disposed near one edge of the circuit board 809 as illustrated in FIG. 8. In the vicinity of the coil antenna 835, therefore, the antenna 825 is formed by being bent toward the inside of the circuit board 809 without following the outer shape of the circuit board 809. This has been problematic in that the area of the opening of the antenna 825 is reduced and the antenna gain is thereby lowered. Another problem has been that if the antenna 825 is disposed near the coil antenna 835 to widen the area of the opening of the antenna 825, mutual coupling occurs between the antenna 825 and the coil antenna 835 and the antenna property of the antenna 825 is deteriorated.